A receiver system typically consists of a series of stages consisting of pre-selectivity gain and mixing, frequency selectivity (i.e. a filter) and post-selectivity gain and mixing. Conventional receivers either set a total system gain with a predetermined partition between pre- and post selectivity gain, or rely on a separate controller or demodulator to independently adjust pre and post selectivity gains to achieve the linearity/noise tradeoff.
FIG. 1 is a simplified block diagram of a receiver 100, as known in the prior art. In receiver 100, amplifier 110 has a gain G1 that provides pre-selectivity gain. Frequency converter 120, which may be a mixer, provides frequency conversion. Filter D1 130 is typically a bandpass filter adapted to filter out undesired signal. Amplifier 140 has a gain of G2 and provides post-selectivity gain. A local oscillator (not shown) is often used to provide an oscillating signal to frequency converter 120. Frequency converter 120, and filter 130 typically have finite linearity and thus it is desirable to limit the range of signals that are coupled to them.
FIG. 2A shows a spectrum of exemplary signals received by filter 130. The desired signal is shown as having the frequency Fd. The spectrum of the receives signals often includes undesired signal components (also referred to as blockers) shown as having frequencies Fb1 and Fb2 that interfere with the desired signal, causing non-linearity, distortion, etc. For example, the spacing and amplitude of the undesired signals Fb1 and Fb2 may result in a third order intermodulation distortion product at the output of amplifier 110. As such, it is not desirable to place too much gain before filter 130 which is adapted to attenuate the blocker signals, as shown in FIG. 2B. The reduction of the undesired signals enables amplifier 140 to amplify the desired frequencies in without substantially increasing the amplitudes of the undesired signals.
By reducing the gain G1 of amplifier 110, the linearity is improved. Reducing the gain of the first amplifier 110 also reduces the amplitude of signal S1. To keep the amplitude of signal S4 constant, gain G2 may be increased. The gain redistribution between amplifiers 110 and 140 reduces distortion but also results in degradation of the signal-to-noise (SNR) ratio. Therefore a tradeoff exists between increasing the gain G1 to improve signal to noise ratio, and degrading linearity performance of the system (increasing the distortion products in the signal) when blockers are present.
Gains G1 and G2 are typically selected such that the total gain G1*G2 is equal to a known value. In accordance with one conventional technique, for a given input signal level S0, a predetermined gain partitioning of G1 and G2 is used. FIG. 3 is a block diagram of a conventional receiver 300 configured to achieve a predetermined gain partitioning of G1 and G2 using control signal Tsys. FIG. 4 shown plots of gains G1, G2 and G1*G2(Gsys) for a receiver having predetermined gain partitions.
In receiver 300, the gains of the first and second amplifiers 110 and 140, respectively, are controlled by gain controller 310 that controls the gains G1 and G2 in accordance with an algorithm that provides fixed gain partitioning using signal Tsys. FIG. 4 shows examples of the gain G1 from amplifier 110, gain G2 from amplifier 140 as well as the products of these two gains. The attack point (AP) represents the signal level at which total gain Gsys begins to be fall. The take-over point (TOP) represents the signal level at which gain control is passed from signal T2 to signal T1. The TOP and AP values are typically predetermined and fixed. In a typical television system, a demodulator is used to generate control signals T1 and T2.
In accordance with another conventional technique, the output signal of the second amplification stage is used to determine the gain partitioning. FIG. 5 is a simplified block diagram of a receiver 500 having gain partitioning controlled by a demodulator 510. Demodulator 510 is configured to controls the values of G1 and G2 depending on the presence and level of blockers. Demodulator 510 operates to control the partitioning of the gain between amplifiers 110 and 140 by sensing the output signal S4 of second amplifier 140. Demodulator 510 may be programmed to estimate whether blockers or other undesired signal components are causing distortion in the desired signal. Demodulator 510 then reparations the gain by adjusting signals T1 and T2.